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<title>Debugging with GDB: All-Stop Mode</title>

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<a name="All_002dStop-Mode"></a>
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Next: <a href="Non_002dStop-Mode.html#Non_002dStop-Mode" accesskey="n" rel="next">Non-Stop Mode</a>, Up: <a href="Thread-Stops.html#Thread-Stops" accesskey="u" rel="up">Thread Stops</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
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<hr>
<a name="All_002dStop-Mode-1"></a>
<h4 class="subsection">5.5.1 All-Stop Mode</h4>

<a name="index-all_002dstop-mode"></a>

<p>In all-stop mode, whenever your program stops under <small>GDB</small> for any reason,
<em>all</em> threads of execution stop, not just the current thread.  This
allows you to examine the overall state of the program, including
switching between threads, without worrying that things may change
underfoot.
</p>
<p>Conversely, whenever you restart the program, <em>all</em> threads start
executing.  <em>This is true even when single-stepping</em> with commands
like <code>step</code> or <code>next</code>.
</p>
<p>In particular, <small>GDB</small> cannot single-step all threads in lockstep.
Since thread scheduling is up to your debugging target&rsquo;s operating
system (not controlled by <small>GDB</small>), other threads may
execute more than one statement while the current thread completes a
single step.  Moreover, in general other threads stop in the middle of a
statement, rather than at a clean statement boundary, when the program
stops.
</p>
<p>You might even find your program stopped in another thread after
continuing or even single-stepping.  This happens whenever some other
thread runs into a breakpoint, a signal, or an exception before the
first thread completes whatever you requested.
</p>
<a name="index-automatic-thread-selection"></a>
<a name="index-switching-threads-automatically"></a>
<a name="index-threads_002c-automatic-switching"></a>
<p>Whenever <small>GDB</small> stops your program, due to a breakpoint or a
signal, it automatically selects the thread where that breakpoint or
signal happened.  <small>GDB</small> alerts you to the context switch with a
message such as &lsquo;<samp>[Switching to Thread <var>n</var>]</samp>&rsquo; to identify the
thread.  
</p>
<p>On some OSes, you can modify <small>GDB</small>&rsquo;s default behavior by
locking the OS scheduler to allow only a single thread to run.
</p>
<dl compact="compact">
<dt><code>set scheduler-locking <var>mode</var></code></dt>
<dd><a name="index-scheduler-locking-mode"></a>
<a name="index-lock-scheduler"></a>
<p>Set the scheduler locking mode.  It applies to normal execution,
record mode, and replay mode.  If it is <code>off</code>, then there is no
locking and any thread may run at any time.  If <code>on</code>, then only
the current thread may run when the inferior is resumed.  The
<code>step</code> mode optimizes for single-stepping; it prevents other
threads from preempting the current thread while you are stepping, so
that the focus of debugging does not change unexpectedly.  Other
threads never get a chance to run when you step, and they are
completely free to run when you use commands like &lsquo;<samp>continue</samp>&rsquo;,
&lsquo;<samp>until</samp>&rsquo;, or &lsquo;<samp>finish</samp>&rsquo;.  However, unless another thread hits a
breakpoint during its timeslice, <small>GDB</small> does not change the
current thread away from the thread that you are debugging.  The
<code>replay</code> mode behaves like <code>off</code> in record mode and like
<code>on</code> in replay mode.
</p>
</dd>
<dt><code>show scheduler-locking</code></dt>
<dd><p>Display the current scheduler locking mode.
</p></dd>
</dl>

<a name="index-resume-threads-of-multiple-processes-simultaneously"></a>
<p>By default, when you issue one of the execution commands such as
<code>continue</code>, <code>next</code> or <code>step</code>, <small>GDB</small> allows only
threads of the current inferior to run.  For example, if <small>GDB</small>
is attached to two inferiors, each with two threads, the
<code>continue</code> command resumes only the two threads of the current
inferior.  This is useful, for example, when you debug a program that
forks and you want to hold the parent stopped (so that, for instance,
it doesn&rsquo;t run to exit), while you debug the child.  In other
situations, you may not be interested in inspecting the current state
of any of the processes <small>GDB</small> is attached to, and you may want
to resume them all until some breakpoint is hit.  In the latter case,
you can instruct <small>GDB</small> to allow all threads of all the
inferiors to run with the <code>set&nbsp;<span class="nolinebreak">schedule-multiple</span></code><!-- /@w --> command.
</p>
<dl compact="compact">
<dd><a name="index-set-schedule_002dmultiple"></a>
</dd>
<dt><code>set schedule-multiple</code></dt>
<dd><p>Set the mode for allowing threads of multiple processes to be resumed
when an execution command is issued.  When <code>on</code>, all threads of
all processes are allowed to run.  When <code>off</code>, only the threads
of the current process are resumed.  The default is <code>off</code>.  The
<code>scheduler-locking</code> mode takes precedence when set to <code>on</code>,
or while you are stepping and set to <code>step</code>.
</p>
</dd>
<dt><code>show schedule-multiple</code></dt>
<dd><p>Display the current mode for resuming the execution of threads of
multiple processes.
</p></dd>
</dl>

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